[Frontend] Restore Preprocessor::getPredefines()

[royjacobson]

Summary

https://reviews.llvm.org/rG6bbf51f3ed59ae37f0fec729f25af002111c9e74 from May removed Preprocessor::getPredefines() from Clang's API, presumably as a cleanup because this method is unused in the LLVM codebase.

However, it was/is used by a small number of third-party tools and is pretty harmless, so this patch adds it back and documents why it's here.

The issue was raised in https://github.com/llvm/llvm-project/issues/57483, it would be nice to be able to land it into Clang 15 as it breaks those third-party tools and we can't easily add it back in bug fix releases.

[brad.king]

LGTM. After applying this patch locally on top of the release/15.x branch, CastXML builds again. Thanks!

[thieta]

LGTM and seems pretty safe so I am not opposed to merge it before 15 final.

I would maybe add a link to the GitHub issue in the comment, but that's a nit.

[royjacobson]

Add a link to github issue.

clang/include/clang/Lex/Preprocessor.h

public:
   const IncludedFilesSet &getIncludedFiles() const { return IncludedFiles; }
 
   /// Return the name of the macro defined before \p Loc that has
   /// spelling \p Tokens.  If there are multiple macros with same spelling,
   /// return the last one defined.
   StringRef getLastMacroWithSpelling(SourceLocation Loc,
                                      ArrayRef<TokenValue> Tokens) const;
 
+  /// Get the predefines for this processor.
+  /// Used by some third-party tools to inspect and add predefines (see
+  /// https://github.com/llvm/llvm-project/issues/57483).
+  const std::string &getPredefines() const { return Predefines; }
+
   /// Set the predefines for this Preprocessor.
   ///
   /// These predefines are automatically injected when parsing the main file.
   void setPredefines(std::string P) { Predefines = std::move(P); }
 
   /// Return information about the specified preprocessor
   /// identifier token.
   IdentifierInfo *getIdentifierInfo(StringRef Name) const {